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Phylum Echinodermata (Spiny skin)

The animals in phylum Echinodermata typically have radial symmetry, i.e. they have no front and back although they clearly have a top and bottom.  Evolutionary theory teaches us that bilateral symmetry developed in order to facilitate hunting: a mobile predator can be more effective if it can focus its attention on where it's going.  Yet, as the photo of the Ochre star, below, shows, the radially symmetrical sea star had no trouble capturing the bilateral chiton!  Most echinoderms, Ochre star included, do indeed move very slowly.  Once again, however, there is always an exception to the rule: the brittle stars can move quite rapidly.  The brittle stars shown below would scurry off so quickly that it took several tries for me to get a decent photo.

(Just for the record, echinderms are not considered truly radial because they go through an early bilateral stage before maturing into the familiar radiate shape.  In fact, biologists believe that echinoderms lost their bilateral symmetry in the course of evolution.)

The most commonly found classes are:

Class Asteroidea (Star shaped): Sea stars

Short-spined star (Pisaster brevispinus)	Ochre star (Pisaster ochraceus) eating a chiton
Six-rayed Star (Leptasterias hexactis)	Sunflower star (Pycnopodia helianthoides)
Pacific Blood Star (Henricia leviuscula)

Class Ophiuroidea (Snake-like): Brittle stars

Brittle stars (Ophiuroidea)
(A southern species is shown here.)

Class Echinoidea (Sea urchin shaped): Sea Urchins and Sand Dollars

Purple Sea Urchin, (Strongylocentrotus purpuratus)

Class Holothuroidea (Sea cucumber shaped): Sea Cucumbers

(no photo)